EP 742 581, for example, relates to a method of making sealed cavities on silicon wafer surfaces by anodic bonding and with electrically insulated conductors through the sealing areas to connect functional devices inside the cavities to electrical terminals outside said cavities. The conductors are provided through the use of doped buried crossings in a single crystal silicon substrate, thereby also allowing production of different kinds of integrated silicon devices, e.g. sensors.
The technique implies that Borosilicate glass plates are bonded onto a silicon (Si) wafer using so-called anodised bonding. A plate of glass is arranged on a Si wafer under an amount of pressure. The Si wafer and the glass are then heated up to some hundred degrees and a voltage is applied across the plates (glass and wafer) whereby the glass, which contains sodium (Na) ions migrate into the Si wafer, and a hermetic junction is obtained.
When two Si wafers are to be connected, a similar method as mentioned above is carried out, however, one of the wafers is coated with sputtered borosilicate glass and an anodised bonding is performed.
Generally, to be able to achieve an anodised bonding the object intended to be bonded to Si must contain Na-ions, usually through doping a glass with Soda lime glass. The reason for using borosilicate glass is that it matches the Si wafer characteristics, specially with respect to coefficient of expansion.
One major problem related to above mentioned and similar methods is the possibility of providing a conducting arrangement through the glass or wafer. In above mentioned European Patent No. 742 581, for example, the electrical connection path is from one of an outside wire bonding area via a first contact diffusion down to a buried conductor which crosses below the sealing area of the cavity, and via a second contact diffusion to a second aluminium interconnection line which establishes connections to two piezo-resistors.